The utilization of secondary air pumps in combination with lambda control processes and catalytic exhaust gas purification is disclosed, for example, in U.S. Pat. No. 4,200,071. In contrast to conventional lambda control systems, the control intervention in the method disclosed in this patent does not operate on the fuel-metering signal, rather, on the air quantity. This takes place by selectively supplying secondary air at the intake end to the precontrolled operating mixture which is slightly rich or by supplying the secondary air at the exhaust gas end to the combustion products of this slightly rich preadjusted mixture. In both cases, an oxygen concentration in the exhaust gas is to be obtained which corresponds to the lambda value of 1 as it is desired for the optimal toxic material conversion in the three-way catalytic converter arranged downstream. For this purpose, it is necessary to maintain the supply of secondary air in at least large portions of the operating phases of the internal combustion engine. This continuous operation is however not desirable because of the noise level and the service life of the secondary air pump.
The lambda control acts primarily on the fuel-metering signal in more modern systems equipped with secondary air pumps. The secondary air pump operates there only in the relatively short time interval of the warm-up phase after a cold start wherein the lambda control is not yet operationally ready. The exothermal reaction of the air, which is blown in between the outlet valves of the engine and the catalytic converter, and the hot exhaust gases and the further oxidation in the catalytic converter lead to an accelerated warm-up of the catalytic converter. The secondary air pump is switched off with the start of the lambda control. One such system is described, for example, in the publication "MTZ" (Motortechnische Zeitschrift), Volume 50 (1989), Number 6, page 249.
The systems operating in accordance with the last-described method however do still have disadvantages. Increased exhaust-gas emissions can occur especially with the restart of an engine, which is still warm, because the temperature of the catalytic converter can drop off rapidly below its operating temperature during an interruption of the engine operation. On the other hand, the danger is present for a warm engine that an operation of the secondary air pump leads rapidly to overheating and therefore to damage of the catalytic converter.